DES MOINES, Iowa (CNN) --Democratic presidential front-runner Howard Dean accused his rivals in debate of being "co-opted" by the Bush administration amid criticism from several of those fellow candidates.

The face-off in Johnston, Iowa, came barely two weeks before the first votes are to be cast in the Iowa caucuses, the first balloting in the Democratic presidential campaign. (CNN.com's interactive Election Calendar)

Dean, who leads among likely Democratic voters in a nationwide CNN/Time poll taken last week, took aim at the support of some Democratic members of Congress for the war in Iraq and President Bush's "No Child Left Behind" education bill, which Democrats accuse Bush of failing to properly fund.

"This is a bill that ought never to have been passed, and this is why I am running for president," Dean said in Sunday's debate.

"What has happened to so many Democrats in Congress is that they've been co-opted by the agenda of George Bush, who came into office with 500,000 fewer votes than Al Gore. And what we need is a Democrat who's going to stand up to George Bush."

Dean's opposition to the war propelled him to the front ranks of the party's 2004 contenders, but Sens. John Kerry of Massachusetts and Joseph Lieberman of Connecticut have criticized several recent statements by Dean.

Those include an off-the-cuff comment about the September 11 attacks and al Qaeda leader Osama bin Laden.

In December, Dean told a newspaper, "I've resisted pronouncing a sentence before guilt is found."

Later, he released a statement saying, "When we capture Osama bin Laden, he will be brought to justice and treated in the same manner that President Bush is recommending for Saddam Hussein."

Another off-the-cuff comment that has drawn criticism from Dean's opponents was his assertion that Americans are no safer with ousted Iraqi leader Saddam Hussein in custody.

"I don't know how anybody could say that we're not safer with a homicidal maniac, a brutal dictator, an enemy of the United States, a supporter of terrorism, a murderer of hundreds of thousands of his own people in prison instead of in power," Lieberman said.

Dean said the deaths of 23 American troops in Iraq since Saddam's capture and the current state of high alert in the United States support his argument.

Rep. Dick Gephardt of Missouri, who has run neck-and-neck with Dean in Iowa polls, questioned Dean's support for free-trade agreements that Gephardt said have costs jobs in Iowa and across the country.

Both Dean and Gephardt said those agreements should include tough labor and environmental standards.

"It's one thing to talk the talk, it's another thing to walk the walk," Gephardt said. "We've got to get labor and environment in these treaties, and we've got to do it when the treaties are before the Congress. That's when it counts."

Lieberman challenged the former Vermont governor to open the records of his governorship he had sealed upon leaving office, offering him a pen to put it in writing.

Dean declined, saying the papers involved personal and legal advice and that a judge would decide what should be made public.

Lieberman, who was the party's 2000 vice presidential nominee, called that answer "unsatisfactory and disappointing."

"You should not force a judge to force you to do what is right," he said.

One late note: CNN has learned that former Sen. Bill Bradley is to endorse Dean Tuesday at an event in New Hampshire, according to sources close to both Dean and Bradley.